1. Field of the Invention
This invention relates to a coating composition. More particularly, this invention relates to a coating composition for improving articles of plastic materials, articles of wooden materials and articles of metallic materials in surface conditions such as surface rigidity, chemical resistance and dyeability.
2. Description of Prior Arts
Shaped articles of plastic materials, i.e. shaped articles of thermoplastic resins such as, for example, polycarbonate, polymethyl methacrylate, polystyrene and polyvinyl chloride excel in transparency, freedom from weightness, ease of fabrication and impact resistance. They nevertheless lack abrasion resistance and solvent resistance and, therefore, tends to sustain scratches on the surface and readily yield to the action of organic solvents. To remedy the drawback, there have been suggested various methods for coating such articles with thermosetting resins. None of these methods, however, have provided any satisfactory remedy although one is needed.
For example, combinations of trialkoxysilanes such as methyl trialkoxysilanes and phenyl trialkoxysilanes and tetraalkoxysilanes such as ethyl silicate and butyl silicate, and mixtures of such combinations with other resinous coating compositions have been suggested as coating compositions. Unfortunately, none of them satisfy all the properties such as abrasion resistance, adhesiveness, hot water resistance, heat resistance and weather resistance. Japanese Laid-open Patent Publication No. 42,752/1976 and Japanese Patent Publication No. 13,818/1977 disclose inventions which involve epoxy group-containing alkoxy-silanes and adopt, as curing catalysts therefore, Lewis acids such as BF.sub.3 and BF.sub.3 etherates or complexes of such acids, Brosted acids such as HCl, HBr, HI, HNO.sub.3, HClO.sub.4 and H.sub.3 PO.sub.4 and metal salts of organic acids such as cobalt naphthenate and zinc naphthenate. These methods, however, are not practicable because the produced coating compositions have a short pot life and require a long curing time. Japanese Laid-open Patent Publication No. 117,529/1974 discloses compositions comprising epoxy group-containing alkoxysilanes and zinc borofluoride, tin borofluoride or boron trifluoride-amine complex. The method taught in this disclosure, however, does not impart ample rigidity to the formed coat. The coat obtained at all by this method is degraded during its exposure to hot water. U.S. Pat. No. 3,986,997 discloses a coating composition comprising coloidal silica and hydrolyzate of methyltrimethoxysilane as a main component. Although the cured coating film has sufficient hardness, it has poor flexibility, and it is very difficult to obtain good adhesiveness to the polycarbonate type resin substrate. Japanese Laid-open Patent Publication No. 111,336/1978 discloses coating compositions comprising one or more of mixtures selected from compounds containing epoxy group and silanol group and/or siloxane group and finely divided silica having a particle diameter of from 1 to 100 milimicrons and an aluminum chelate compound, but the cured coated film has poor hardness and the hardness is impaired by boiling, dipping into water and the like, and further, it exhibits poor adhesiveness to diethylene glycol (CR-39) bisphenol type polycarbonate substrate.
Further, when the surface coating treatment is applied to a plastic eye glass lens and the like, it is desired to be easy to dye the formed coated film, but the coated film having surface hardness has generally poor dyeability. Thus, according to usual dyeing method which is widely used to dye fibers and plastics, it is impossible to dye or even if it is possible to dye, the coated film decreases in hardness and performances on account of severe dyeing conditions. U.S. Pat. No. 4,173,490 discloses a coating composition which comprises a cohydrolyzate of ethyl silicate and alkyl trialkoxysilane and/or dialkyl dialkoxysilane modified with a siloxane having active polar group or a nonionic surface active agent in order to be able to dye, but the cured coated film is easy to crack and has poor dyeability.
It is therefore, an object of this invention to provide a coating composition which excels in abrasion resistance, hot water resistance, adhesiveness and weather resistance and enjoys a long pot life.